A known dual-battery system includes a vehicle electrical system battery and a starter battery that are connected to one another when the generator is operating, e.g., see: German Patent 38 41 769. When generator current delivery is low, this has the disadvantage that the vehicle electrical system battery is charged by charge exchange at the expense of the starter battery, if the charge state of the vehicle electrical system battery is less than that of the starter battery. With the known dual battery system, a switch is inserted between the vehicle electrical system battery (which carries the vehicle electrical system loads) and the remainder of the vehicle electrical system. That switch is opened, when the generator is not running as soon as the starter battery voltage falls below a permissible limit value. This feature protects the starter battery from complete discharge only when the generator is not running and also allows partial discharge of the starter battery. This poor condition (partial discharge) increases the initial load on the starter battery as vehicle travel begins and a charge exchange between the two batteries may take place to the detriment of the starter battery. In extreme cases, this can lead to complete discharge or destruction of the starter battery.
German Patent 40 28 242 discloses a dual-battery system in which the connection between starter battery and vehicle electrical system battery can be broken, regardless of the driving status, if the vehicle electrical system battery would otherwise be charged from the starter battery. The two batteries are connected to one another if the vehicle electrical system battery voltage is greater than the starter battery voltage. If the voltage of the vehicle electrical system battery is less than that of the starter battery, the two batteries are disconnected from one another. The disconnection can be performed in various ways, for example by cutting-off the starter battery from the rest of the vehicle electrical system. As a result, the starter battery is not subjected to the load of a discharged vehicle electrical system battery, and in working mode the generator can power the most important loads while charging the vehicle electrical system battery. The two batteries can also be disconnected from the rest of the vehicle electrical system by cutting off the vehicle electrical system battery. If an optional connection is provided between the generator and the starter battery the starter battery can be charged.
A disadvantage of the known dual-battery systems is that activation of the starter battery switch by the control device is very complex. For example, the charge states of the vehicle electrical system battery and the starter battery must be sensed in order to determine the control information.